1. Field of the Invention
The present invention relates to a magnetic resistance device used in magnetic heads and various types of sensors, and more particularly, to a magnetic resistance device in which a pair of magnetic tunnel junction structures, each composed of a barrier film sandwiched between a lower magnetic layer and an upper magnetic layer, formed connected in series on a substrate.
2. Description of Related Art
Magnetic resistance devices utilizing anisotropic magneto-resistance effects such as NiFe, and magnetic resistance devices that obtain giant magneto-resistance effects (GMR effects) utilizing the scattering of electrons that is dependent on their spin, are widely known in the prior art. Magnetic resistance change rate is one parameter used to evaluate the properties of this type of magnetic resistance device, and this magnetic resistance change rate exhibits a value of about 3% in the case of the former, and about 10% in the case of the latter.
Magnetic resistance devices have appeared in recent years that utilize magnetic tunnel effects to demonstrate large values for magnetic resistance change rate of 20–30%. As is indicated in, for example, Japanese Patent Application, First Publication (Kokai), No. Hei 11-135857, in addition to forming a magnetic tunnel junction structure, in which a barrier film comprised of an insulating material (e.g. Al2O3) is sandwiched between a lower magnetic layer comprised of a ferromagnetic material (e.g. Ni0.81Fe0.19 or CoFe) and an upper magnetic layer comprised of a ferromagnetic layer (e.g. Ni0.81Fe0.19), on a substrate, these magnetic resistance devices provide an antiferromagnetic layer (e.g. FeMn) directly beneath the lower magnetic layer, and use the lower magnetic layer as a stationary layer and the upper magnetic layer as a free layer to obtain a higher magnetic resistance change rate due to magnetic tunnel effects. In addition, even if an antiferromagnetic layer is not provided, magnetic tunnel effects similar to those described above are also known to be realized by providing a difference in coercive force between the upper magnetic layer and lower magnetic layer.
When using a magnetic resistance device using this magnetic tunnel effect, although a voltage is obtained corresponding to the magnetic resistance change rate relative to the change in the magnetic field by applying a constant current to the magnetic resistance device (magnetic tunnel junction), in the case of this magnetic resistance device, when the applied voltage is increased, the magnetic resistance change rate tends to decrease gradually. Together with this decrease, the decreasing trend in the magnetic resistance change rate becomes more remarkable the larger the resistance value. Thus, when a large voltage is applied to a magnetic resistance device, the problem is encountered in which it is not possible to obtain an adequate detection voltage corresponding to the change in the magnetic field since this large voltage causes a large decrease in the magnetic resistance change rate. Conversely, when a magnetic resistance device having a small resistance value is used, although the above-mentioned tendency for the magnetic resistance change rate to decrease is small, since the resistance of the magnetic resistance device itself is small, the same problem is encountered in which it is not possible to obtain a sufficiently large voltage.
In order to deal with these problems, as is indicated in, for example, Japanese Patent Application, First Publication (Kokai), No. Hei 11-97762, and Japanese Patent Application (Kokai), First Publication, No. Hei 11-112054, the use of a plurality of magnetic tunnel junction structures connected in series has been proposed. In this case, in the case of the former, a pair of barrier films and upper magnetic layers are independently and separately provided on a continuously formed lower magnetic layer so that use of the lower magnetic layer is shared. In addition, in the case of the latter, a plurality of basic units, each consisting of a tunnel junction structure in which separate pairs of barrier films and upper magnetic layers are formed on a common lower magnetic layer as described above, are arranged horizontally, and coupled with a continuous upper magnetic layer that is shared by adjacent basic units so as to connect a large number of tunnel junctions in series using common lower and upper magnetic layers.
In the case of forming a plurality of magnetic tunnel junction structures comprising a barrier film sandwiched between a lower magnetic layer and upper magnetic layer on a substrate and connecting in series as described above, fine machining processing is required, and wet etching is not suitable for the production of such elements. Thus, machining processing using dry etching in the form of ion beam etching is suitable for a laminate consisting of a lower magnetic layer, barrier film and upper magnetic layer on a substrate. However, since it is necessary to hold the surface roughness of the lower magnetic layer to a low level and it is not possible to compose a thick lower magnetic layer, it is difficult to reliably separate the barrier film while sharing a lower magnetic layer by ion beam etching. Namely, in the case of removing the barrier film by ion beam etching, although it is possible to control completion of etching treatment by measuring a prescribed time or detecting the material that composes the lower layer of the barrier film and so forth, if etching treatment is terminated too soon, the pair of barrier films cannot be reliably separated. As a result, this resulted in the problems of poor yield in the production of magnetic resistance devices and the occurrence of large variations in properties.
In addition, examples of major resistance elements in magnetic resistance devices using the magnetic tunnel effect as described above include electrodes and other wiring components as well as the barrier film and lower magnetic layer. Since only the barrier film of these resistance elements changes relative to changes in the magnetic field, in order to obtain a large magnetic resistance change rate, it is necessary to lower the resistance values of resistance elements other than the barrier film. In particular, if the lower magnetic layer located beneath barrier film is composed to be thick, the surface roughness of the same lower magnetic layer and so forth increases. Since this causes the formation of pin holes in barrier film, the lower magnetic layer and so forth cannot be excessively thick, and in the case current flowing between each magnetic tunnel junction structure flows through a common lower magnetic layer and so forth, the resistance element of this lower magnetic layer becomes a problem. However, in descriptions relating to the above-mentioned magnetic resistance devices of the prior art, only the connection of magnetic tunnel junction structures in series is presented, while no considerations whatsoever have been given to an optimum connection method, thereby preventing the obtaining of a high magnetic resistance change rate.